1. Technical Field
The invention relates to a semiconductor device that can be used as a light-emitting device and to a method of manufacturing such device. In particular, the invention relates to a semiconductor light-emitting device and its manufacturing method, where the confinement structure and low resistance areas are easily formed, the reproducibility thereof is superior, and the yield thereof is very high.
2. Description of the Prior Art
As shown on FIG. 1, a semiconductor laser having a layered structure is formed from a substrate 70, n-type cladding layer 71, active layer 72, and p-type cladding layer 73. A top electrode 74 and a bottom electrode 75 are placed at the two ends of this layered structure.
When current is injected into the active layer 72 from the top electrode 74, light resonance arises in the active layer 72 and laser light 76 is emitted in a prescribed direction, e.g. in a perpendicular direction for vertical cavity surface emitting lasers. Because current is supplied to the active layer 72 from the top electrode 74, the p-type cladding layer 73 must have a low resistivity (generally, about 1.OMEGA..multidot.cm). Japanese Unexamined Patent Publication No. 5-183189 disclosed a technology that may be used to make a low resistivity p-type cladding layer 73.
In the semiconductor laser device having the form shown on FIG. 1, the p-type cladding layer 73 extends over the entire structure and has uniform resistivity. Accordingly, the following problems arise:
Light emission efficiency decreases; PA1 Threshold current at the start of oscillation becomes large; PA1 The device is easily destroyed by the generation of heat; and PA1 During oscillation operation the device becomes unstable. PA1 In continuous irradiation, the temperature distribution in the sample attains a steady state. The temperature reaches its maximum in the part of the device that absorbs the laser light, and becomes lower in the part of the device with no absorption away from the absorption part. PA1 In contrast, in pulsed irradiation the temperature distribution in the sample changes over time. At the instant of the pulsed irradiation, the temperature in the absorption part increases, but it takes a relatively long time until the heat generated by the absorption diffuses. Therefore, the temperature surrounding the absorption part does not increase immediately. As a result, an extremely steep temperature gradient can occur at the boundary between the absorption region and the non-absorption region.Consequently, for pulsed irradiation, a more abrupt change in resistivity can be created between the absorption region and non-absorption region than in continuous irradiation. By adjusting the strength and pulse width of the laser light, a variety of resistivity gradients can be formed.
A known solution to the above problems is to use a conventional current-confined semiconductor laser device, as shown in FIGS. 2(A) to 2(C). The laser device in FIG. 2(A) has a planar stripe structure in which an n-type contact layer is formed on a p-type cladding layer; and in which a top electrode is formed after Zn that is diffused in a striped form reaches the p-type cladding layer through the n-type contact layer. The structure is referred to as a confinement type structure, although the current spread in such device is large and the degree of confinement is poor.
The laser device shown on FIG. 2(B) has a proton implanted structure. In this device, a p-type contact layer is formed on a p-type cladding layer. The top electrode is formed in a striped part that remains after protons are implanted. To make this structure, it is necessary to control the amount of implantation of the protons. Because this process exhibits inferior reproducibility, it is not easy to use the process to fabricate devices of uniform quality.
The laser device shown on FIG. 2(C) has a buried heterostripe structure and, as such, it is layered with an n-type cladding layer, an active layer, a p-type cladding layer on an n-type substrate (InP), a top electrode, and a bottom electrode. This structure exhibits excellent current confinement because the p-type cladding layer has a confinement structure. However, in forming the confinement structure, manufacturing becomes complex because etching and regrowth are essential steps.
For laser devices, such as those shown on FIGS. 2(A) to 2(C), where the laser device has a current-confined structure, processing cannot usually be repeated when a processing error occurs. Therefore, such processes as are used to produce these devices result in poor yields and an accompanying negative effect on manufacturing costs.
Except for the structure of FIG. 2(C), generally, when a structure that confines the current in the light emitting area is formed in a laser device, the bonding area between the contact layer and electrode layer necessarily becomes narrow. As a result, the contact resistance between the semiconductor and metal becomes large, Joule heat arises in the contact while the element is operating, and the characteristics of the element deteriorate.
An annealing method that consists of heating by a heater and electron beam radiation, in addition to laser light radiation, produces problems related to local heating when the device is heated by the heater. Consequently, a method other than local heating must be devised to form the current-confinement structure. For electron beam radiation, local heating is possible. However, because the electron beam scans, annealing by this technique takes considerable time and therefore significantly reduces process productivity.